<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        
        ul {
            width: 400px;
            height: 400px;
            background: skyblue;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
        }
        
        ul li {
            width: 50px;
            height: 50px;
            background-color: slateblue;
            list-style: none;
        }
    </style>
</head>

<body>
    <ul>
        <li></li>
        <li></li>
        <li></li>
    </ul>
</body>

</html>